Selectins are plasma membrane carbohydrate-binding adhesion molecules that mediate an initial step of low-affinity adhesion of circulating leukocytes to endothelial cells lining postcapillary venules (Table 1). The extracellular domains of selectins are similar to C-type lectins, so called because they bind carbohydrate structures (lectins are carbohydrate binding proteins) in a calcium-dependent manner. Selectins and their ligands are expressed on leukocytes and endothelial cells. Importantly, the expression of selectins and their ligands is increased by exposure to cytokines produced at sites of tissue injury and infection, and this is one mechanism by which leukocytes are recruited to sites of inflammation.

Table1. Major Leukocyte-Endothelial Adhesion Molecules

Endothelial cells express two types of selectins, called P-selectin (CD62P) and E-selectin (CD62E). P-selectin, so named because it was first found in platelets, is stored in cytoplasmic granules of endothelial cells and is rapidly redistributed to the luminal surface in response to histamine from mast cells and thrombin generated during blood coagulation. E-selectin is synthesized and expressed on the endothelial cell surface within 1 to 2 hours in response to the cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF), which are produced by tis sue sentinel cells (DCs and macrophages) in response to infection. Microbial products such as lipopolysaccharide (LPS) also stimulate E-selectin expression on endothelial cells. We will describe IL-1, TNF, and LPS in our discussion of inflammation in Chapter 4.

The ligands on leukocytes that bind to E-selectin and P-selectin on endothelial cells are complex sialylated carbohydrates related to the Lewis X or Lewis A family of blood group molecules. These chemical structures are present on various sur face glycoproteins of granulocytes, monocytes, and some previ ously activated effector and memory T cells. The best defined of these is the tetrasaccharide sialyl Lewis X (the structure was originally identified as an antigen on red blood cells in studies of genetically related people with the family name Lewis). A leukocyte membrane glycoprotein called P-selectin glycoprotein ligand 1 (PSGL-1) is posttranslationally modified to display sialyl Lewis X, which serves as the major carbohydrate ligand for P-selectin. Several different molecules may display the carbohydrate ligand for E-selectin, including the glycoproteins PSGL-1 and E-selectin ligand-1 and some glycolipids.

A third selectin, called L-selectin (CD62L), is expressed on leukocytes and not on endothelial cells. The ligands for L-selectin are sialomucins on endothelial cells, whose expression may be increased by cytokine activation of the cells. A major recognition determinant that L-selectin binds to on these sialomucins is 6-sulfo sialyl Lewis X. The expression of the ligand on endothelial cells is increased by IL-1, TNF, and other inflammatory cytokines, so L-selectin on neutrophils promotes the adhesion of these cells to endothelial cells at sites of inflammation. In adaptive immunity, L-selectin is required for naive T and B lymphocyte homing into lymph nodes through specialized blood vessels called high endothelial venules (HEVs). The sialomucin ligands on HEVs that bind to L-selectin on naive lymphocytes are collectively called peripheral node addressin (PNAd). Leukocytes express L-selectin and the carbohydrate ligands for P-selectin and E-selectin at the tips of their micro villi, facilitating interactions with molecules on the endothelial cell surface.

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